U.S. patent number 7,162,049 [Application Number 10/337,347] was granted by the patent office on 2007-01-09 for ported loudspeaker system and method with reduced air turbulence, bipolar radiation pattern and novel appearance.
This patent grant is currently assigned to Britannia Investment Corporation. Invention is credited to Matthew S Polk, Jr..
United States Patent |
7,162,049 |
Polk, Jr. |
January 9, 2007 |
Ported loudspeaker system and method with reduced air turbulence,
bipolar radiation pattern and novel appearance
Abstract
A loudspeaker system includes a cabinet with an interior air
volume, a transducer, a first port extending from an opening in the
front wall of the cabinet to the interior of the cabinet, and a
second port extending from and opening in the rear wall of the
cabinet to the interior of the cabinet. The first and second ports
are aligned along a common longitudinal axis and the interior ends
of the ports are separated from each other by a predetermined
distance. First and second flanges having a diameter larger than
the first and second ports are disposed at the interior ends of the
first and second ports, respectively.
Inventors: |
Polk, Jr.; Matthew S (Gibson
Island, MD) |
Assignee: |
Britannia Investment
Corporation (San Diego, CA)
|
Family
ID: |
32681226 |
Appl.
No.: |
10/337,347 |
Filed: |
January 7, 2003 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20040131219 A1 |
Jul 8, 2004 |
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Current U.S.
Class: |
381/345;
181/156 |
Current CPC
Class: |
H04R
1/2826 (20130101); H04R 1/2819 (20130101) |
Current International
Class: |
H04R
1/28 (20060101) |
Field of
Search: |
;381/335,186,337,338,342,345,351,163,357,180,349,350,389,71.4,71.7,146
;181/163,147,156,145,160,199,144,155 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tran; Sinh
Assistant Examiner: Briney, III; Walter F
Attorney, Agent or Firm: Sterne, Kessler, Goldstein &
Fox, P.L.L.C.
Claims
What is claimed is:
1. A loudspeaker system comprising: an enclosure including a first
exterior wall, a second exterior wall disposed opposite the first
exterior wall, and an interior, wherein the first and second
exterior walls separate the interior of said enclosure from an
exterior of the loudspeaker system; a transducer at least partially
disposed within the interior of said enclosure; a first port
extending from an opening in the first exterior wall to an end of
said first port in the interior of said enclosure; and a second
port extending from an opening in the second exterior wall to an
end of said second port in the interior of said enclosure, wherein
said transducer is at least partially disposed in an interior air
space that is the same interior air space occupied by the interior
ends of said first and second ports, and wherein the respective
ends of said first port and said second port are separated by a
predetermined distance within the interior of said enclosure, such
that acoustic energy from the interior air space exits said
enclosure through said first and second ports approximately
simultaneously and air in said first and second ports and between
the interior ends of said first and second ports operates
substantially as a single acoustic mass.
2. The loudspeaker system of claim 1, further comprising: a first
flange disposed at the end of said first port in the interior of
said enclosure; and a second flange disposed at the end of said
second port in the interior of said enclosure.
3. The loudspeaker system of claim 2, wherein said first port and
said second port have a first diameter and said first flange and
said second flange have a second diameter larger than said first
diameter.
4. The loudspeaker system of claim 2, wherein the predetermined
distance separating the respective ends and flanges of said ports
is less than approximately a diameter of said first and second
ports.
5. The loudspeaker system of claim 2, wherein said first flange
extends around the end of said first port in the interior of said
enclosure and generally away from a central axis of said first
port, and said second flange extends around the end of said second
port in the interior of said enclosure and generally away from a
central axis of said second port, such that a cross-sectional area
between said first and second flanges is larger than a
cross-sectional area of either of the first or second ports.
6. The loudspeaker system of claim 1, wherein said first port and
said second port are aligned on a common axis.
7. The loudspeaker system of claim 1, wherein said first port and
said second port are arranged such that there is an unobstructed
view from the exterior of the loudspeaker system, through the
opening in said first exterior wall through the interior of the
enclosure to the opening in said second exterior wall.
8. The loudspeaker system of claim 1, further comprising a flow
guide disposed in the interior of said enclosure, said flow guide
being located between the ends of said first port and said second
port.
9. The loudspeaker system of claim 1, wherein said first port and
said second port have substantially the same length.
10. The loudspeaker system of claim 1, wherein said first port and
said second port are substantially circular in cross-section.
11. The loudspeaker system of claim 1, wherein said first and
second ports have a diameter, and the predetermined separation
distance between said first and second ports is approximately 1/2
of the diameter of said first and second ports.
12. A loudspeaker system comprising: a transducer; an enclosure
including a first exterior wall, a second exterior wall disposed
opposite the first exterior wall, and an interior, wherein the
first and second exterior walls separate the interior from an
exterior of the loudspeaker system; a first port extending from an
opening in the first exterior wall to an end of said first port in
the interior of said enclosure; and a second port extending from an
opening in the second exterior wall to an end of said second port
in the interior of said enclosure, wherein the respective ends of
said first port and said second port are separated by a
predetermined distance within the interior of said enclosure such
that the total acoustic radiation pattern from the first port and
the second port is approximately bipolar and air in said first and
second ports and between said interior ends of said first and
second ports operates substantially as a single acoustic mass.
13. The loudspeaker system of claim 12, further comprising: a first
flange disposed at the end of said first port in the interior of
said enclosure; and a second flange disposed at the end of said
second port in the interior of said enclosure.
14. The loudspeaker system of claim 13, wherein said first port and
said second port have a first diameter and said first flange and
said second flange have a second diameter larger than said first
diameter.
15. The loudspeaker system of claim 13, wherein the predetermined
distance separating the respective ends and flanges of said ports
is less than approximately a diameter of said first and second
ports.
16. The loudspeaker system of claim 13, wherein said first flange
extends around the end of said first port in the interior of said
enclosure and generally away from a central axis of said first
port, and said second flange extends around the end of said second
port in the interior of said enclosure and generally away from a
central axis of said second port, such that a cross-sectional area
between said first and second flanges is larger than a
cross-sectional area of either of the first or second ports.
17. The loudspeaker system of claim 12, wherein said first port and
said second port are aligned on a common axis.
18. The loudspeaker system of claim 12, wherein said first port and
said second port are arranged such that there is an unobstructed
view from the exterior of the loudspeaker system, through the
opening in said first exterior wall through the interior of the
enclosure to the opening in said second exterior wall.
19. The loudspeaker system of claim 12, further comprising a flow
guide disposed in the interior of said enclosure, said flow guide
being located between the ends of said first port and said second
port.
20. The loudspeaker system of claim 12, wherein said first port and
said second port have substantially the same length.
21. The loudspeaker system of claim 12, wherein said first port and
said second port are substantially circular in cross-section.
22. The loudspeaker system of claim 12, wherein said first and
second ports have a diameter, and the predetermined separation
distance between said first and second ports is approximately 1/2
of the diameter of said first and second ports.
23. A loudspeaker system comprising: a transducer; an enclosure
including a first exterior wall, a second exterior wall disposed
opposite the first exterior wall, and an interior, wherein the
first and second exterior walls separate the interior of the
enclosure from an exterior of the loudspeaker system; a first port
extending from an opening in the first exterior wall to an end of
said first port, wherein the end of said first port includes a
flange located in the interior of said enclosure; and a second port
extending from an opening in the second exterior wall to an end of
said second port, wherein the end of said second port also includes
a flange located in the interior of said enclosure and oriented to
oppose the flange of the first port, wherein the respective ends
and flanges of said first and second ports are separated by a
predetermined distance within the interior of said enclosure, such
that air in said ports and between the respective ends and flanges
of said first and second ports operates as a single acoustic
mass.
24. The loudspeaker system of claim 23, wherein the predetermined
distance separating the respective ends and flanges of said first
and second ports is less than approximately a diameter of said
first and second ports.
25. The loudspeaker system of claim 24, wherein said first port and
said second port have a first diameter and said first flange and
said second flange have a second diameter larger than said first
diameter.
26. The loudspeaker system of claim 23, wherein said first port and
said second port are aligned on a common axis.
27. The loudspeaker system of claim 23, wherein said first port and
said second port are arranged such that there is an unobstructed
view from the exterior of the loudspeaker system, through the
opening in said first exterior wall through the interior of the
enclosure to the opening in said second exterior wall.
28. The loudspeaker system of claim 23, further comprising a flow
guide disposed in the interior of said enclosure, said flow guide
being located between the ends of said first port and said second
port.
29. The loudspeaker system of claim 23, wherein said first port and
said second port have substantially the same length.
30. The loudspeaker system of claim 23, wherein said first port and
said second port are substantially circular in cross-section.
31. The loudspeaker system of claim 23, wherein said first flange
extends around the end of said first port in the interior of said
enclosure and generally away from a central axis of said first
port, and said second flange extends around the end of said second
port in the interior of said enclosure and generally away from a
central axis of said second port, such that a cross-sectional area
between said first and second flanges is larger than a
cross-sectional area of either of the first or second ports.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to loudspeaker systems and in
particular relates to an improved loudspeaker having a unique port
or vent geometry together with a corresponding method of porting a
loudspeaker in an efficient manner and with a novel appearance.
2. Related Art
Vented box loudspeaker systems have been popular for at least 50
years as a means of obtaining greater low frequency efficiency from
a given cabinet volume. Significant advances were made in
understanding and analyzing vented loudspeaker systems through the
work of Thiele and Small during the 1970's. Since then, readily
available computer programs have made it possible to easily
optimize vented loudspeaker designs. However, practical
considerations often prevent these designs, optimized in theory,
from being realized in actuality or from functioning as
intended.
There are two basic approaches in common use in connection with
vented loudspeaker systems, these being the ducted port and the
passive radiator. Although the passive radiator approach has some
advantages, the ducted port has been, in general, more popular due
to lower cost, ease of implementation and generally requiring less
space.
There are, however, disadvantages to the ducted port approach.
These relate principally to undesirable noise and attendant losses
which may be generated by the port at the higher volume velocity of
air movement required to produce higher low frequency sound
pressure levels. For example, as is well known to those skilled in
the art, a vented loudspeaker system has a specific tuning
frequency, fp, determined by the volume of air in the enclosure and
the acoustic mass of air provided by the port according to the
relationship;
.pi..times. ##EQU00001## where MAP is the acoustic mass of the port
and CAB is the compliance of the air in the enclosure. In general,
a lower tuning frequency is desirable for higher performance
loudspeaker systems. As can be seen, either greater acoustic mass
in the port or greater compliance resulting from a larger enclosure
volume is required to achieve a lower tuning frequency. The
acoustic mass of a port is directly related to the mass of air
contained within the port but inversely related to the
cross-sectional area of the port. This suggests that to achieve a
lower tuning frequency a longer port with smaller cross-sectional
area should be used. However a small cross-section is in conflict
with the larger volume velocities of air required to reproduce
higher sound pressure levels at lower frequencies. For example, if
the diameter of a port is too small or is otherwise improperly
designed, non-linear behavior such as chuffing or port-noise due to
air turbulence can result in audible distortions and loss of
efficiency at low frequencies particularly at higher levels of
operation. In addition, viscous drag from air movement in the port
can result in additional loss of efficiency at lower frequencies.
Increasing the cross-sectional area of a port can reduce turbulence
and loss but the length of the port must be increased
proportionally to maintain the proper acoustic mass for a given
tuning frequency. The required increase in length, however, may be
impractical to implement. Other difficulties may also arise as the
length of the port and cross-section are increased. Organ pipe
resonances occur in open-ended ducts at a frequency which is
inversely proportional to the length of the duct. These organ pipe
resonances may produce easily audible distortion when they occur
within certain ranges of frequencies. For example a duct nine
inches in length will have a highly audible principle resonance at
approximately 700 Hz while a duct only 3 inches in length would
have a much less audible principle resonance at approximately 2,100
Hz. In fact, a typical strategy employed in the design of vented
loudspeaker systems is the use of shorter ports such that the organ
pipe resonances occur at higher frequencies where they are less
audible and less likely to be within the range of the transducers
mounted in the enclosure. In addition, a larger cross-sectional
area may lead to undesirable transmission of mid-range frequencies
generated inside the enclosure to the outside of the enclosure.
This may also lead to audible distortion in the form of frequency
response variations due to interference with the direct sound
produced by the loudspeaker system.
Therefore, the design of ports for vented loudspeaker systems
involves conflicting requirements. A large cross-sectional area is
required to avoid audible noise and losses due to non-linear
turbulent flow but this makes it difficult to achieve the acoustic
mass required for a low tuning frequency within practical size
constraints. As will be familiar to those skilled in the art,
various methods have been employed to construct ports with reduced
turbulence and loss. One such example is shown in FIG. 1, which is
a cross-sectional view of a loudspeaker enclosure 100 including a
transducer 102 and a port 104 that is flared at one or both ends of
the port in order to reduce turbulence. The flared port 104
operates to reduce turbulence by increasing the cross-sectional
area of the port at one or both ends thereby slowing the particle
velocity of air at the exits. This allows for a smaller
cross-section in the middle section of the port and a higher
acoustic mass for a given length. However, in order to be
effective, the required flared ends 106, 108 may be quite large and
may, themselves, add significantly to the overall port length
without significantly contributing to the acoustic mass. The
increased cross-section of the flare may increase the transmission
of undesirable midrange frequencies from inside the loudspeaker
cabinet and an improperly selected rate of flare may actually
increase turbulence.
Another conventional method used to decrease turbulence and loss is
shown in FIG. 2, which is a cross-sectional view of a loudspeaker
enclosure 200 with a transducer 102 and multiple ports 204 and 206.
Using multiple ports 204 and 206 decreases turbulence and loss by
taking advantage of the combined cross-sectional area of several
ports. However, as with a single port, the length of each of the
multiple ports must be increased to account for the greater total
cross-section. For example, if two identical ports are used they
will both need to be approximately twice as long as a single port
of the same cross-section to achieve the same acoustic mass and
tuning frequency. As discussed above this may lead to impractical
length requirements and more audible organ pipe resonances.
Other techniques are also used to reduce turbulence and loss as
well as the other difficulties associated with the design of ports
as previously discussed. These include ports with rounded or
flanged ends, geometries to reduce organ pipe resonances and a
plethora of methods for implementing longer ports through folding
or other convolutions.
U.S. Pat. Nos. 5,517,573 and 5,809,154 to Polk, et al.,
incorporated herein in their entirety by reference, disclose
improved porting methods for achieving the required acoustic mass
in a compact space with reduced turbulence and loss. FIG. 3 is a
reproduction of FIG. 7 from the '573 patent. The method described
in these patents involves the use of a disk at the end or ends of a
simple duct to effectively create an increasing cross-sectional
area at the ends of the port. In some preferred embodiments flow
guides are also used to further improve the efficiency of the port
structure. This method has the advantages of suppressing
transmission of midrange frequencies from inside the cabinet and of
providing the required acoustic mass in a more compact form which
also reduces turbulence and loss.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved porting
arrangement and method for use in a loudspeaker system with reduced
turbulence and loss, reduced transmission of midrange frequencies
and less audible organ pipe resonances.
It is another object of this invention to provide an efficient port
structure with a novel appearance which is more compact, simpler to
implement and which has a bipolar radiation pattern.
Briefly and in accordance with one embodiment of the present
invention, a first port is provided in the speaker baffle of the
loudspeaker system with a predetermined length extending inwardly
into the speaker cabinet. A second port is provided in the opposite
wall of the loudspeaker enclosure from the speaker baffle of
similar cross-section to the first port with a predetermined length
extending inwardly into the speaker cabinet toward the first port
and aligned on a common axis with the first port such that the
inward ends are separated by a predetermined separation distance
inside the loudspeaker enclosure and such that the two ports
together appear to provide an unobstructed open duct passing
entirely through the loudspeaker cabinet from front to back. The
additional acoustic mass required to achieve a desired tuning
frequency is provided by flanges of a predetermined diameter,
greater than the ports, affixed concentrically to the inward end of
each of the ports and separated by a predetermined separation
distance. The two flanges or disks provide a circumferential
extension of the internal separation distance between the two
ports. The effect of this arrangement is to provide an increasing
cross-sectional area at the inside end of the port structure for
the purpose of reducing turbulence and loss. Mid-range transmission
from the interior of the loudspeaker cabinet is suppressed since
higher frequencies will tend to pass through the separation between
the two ports with very little midrange energy escaping through the
ports to the exterior of the loudspeaker cabinet. The principle
organ pipe resonance due to the combined length of the ports is
also suppressed due to the separation distance between the two
ports. Due to the front and back openings, the port structure of
the present invention will also have a radiation pattern which is
approximately bipolar at low frequencies. Bipolar radiation of
sound refers to the radiation of in-phase acoustic energy from both
front and back of a loudspeaker system in similar but not
necessarily equal amounts. Bipolar radiation of sound is believed
to result in a more even distribution of low frequency energy into
the listening area. In addition, the two port openings provide a
larger cross-sectional area which further reduces turbulence and
loss. Finally, the illusion of an unobstructed duct passing
entirely through the loudspeaker enclosure presents a novel
appearance.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
FIG. 1 is cross-sectional view of a vented loudspeaker having a
flared port.
FIG. 2 is cross-sectional view of a vented loudspeaker having
multiple ports.
FIG. 3 is a cross-sectional view of a vented loudspeaker woofer
having a port geometry in accordance with the principles of U.S.
Pat. No. 5,517,573.
FIG. 4 is cross-sectional view of vented loudspeaker having a port
geometry in accordance with the principles of the present
invention.
FIG. 5 is a cross-sectional view of a vented loudspeaker having a
port geometry in accordance with the principles of the present
invention, including discs at the outer openings of the port
tubes.
FIG. 6 is a cross-sectional view of a vented loudspeaker having a
port geometry in accordance with the principles of the present
invention and including a flow guide therein.
DETAILED DESCRIPTION OF THE INVENTION
As discussed above, there are various tradeoffs involved in the
design of ducted ports for a loudspeaker system. Increases in
cross-sectional area required to reduce turbulence and loss require
increases in port length to achieve the required acoustic mass. The
increased port length may be too large for the system dimensions
and may also lead to organ pipe resonances at frequencies more
likely to cause audible problems. Use of flared ends as part of the
port structure, as shown in FIG. 1, may reduce turbulence and loss
for a given cross-sectional area in the central part of the port,
but the flared ends themselves contribute little to the required
acoustic mass while making the port structure substantially larger.
As noted above, U.S. Pat. Nos. 5,517,573 and 5,809,154 to Polk, et
al. disclose a porting method and arrangement for reducing
turbulence and loss which is more compact and offers certain other
advantages in suppressing unwanted midrange transmission and organ
pipe resonances.
The present invention uses a novel method and arrangement to
achieve additional benefits and advantages over the prior art.
Referring to FIG. 4, a loudspeaker system is shown composed of an
enclosure or cabinet 400 with at least one transducer 102 mounted
on a speaker baffle 402. A first port tube 404 of inside diameter
D1 and length L is provided on speaker baffle 402 with an outer
opening 406, and a second port tube 408 of inside diameter D1 and
length L, with outer opening 410, is provided on a rear wall 412 of
enclosure 400 opposite speaker baffle 402 such that the two ports
are on a common axis 414 and appear to provide an unobstructed open
duct passing entirely through the loudspeaker enclosure from front
to back. The length L of each of first and second port tubes 404,
408 is selected so as to provide a predetermined separation
distance S between inside ends of the two port tubes. Circular
flanges 416 and 418 of an outside diameter D2 that is greater than
inside diameter D1, are affixed as shown to the inside ends of port
tubes 404 and 408, respectively.
Considered together and as a whole, the port structure shown in
FIG. 4 provides a ducted path with a circumferential opening 420
between outer ends 424, 426 of flanges 416, 418, respectively,
inside the loudspeaker enclosure 400, and two outside openings 406
and 410, in the speaker baffle 402 and rear wall 412, respectively.
The port structure contains the air volume between the two flanges
416 and 418, and the air volume in the two port tubes 404 and 408.
The entire air volume contained by the port structure is intended
to function as a single acoustic mass in determining the tuning
frequency of the system. In the case of substantially identical
port tubes 404 and 408, the acoustic mass of the port structure is
equal to approximately one half the acoustic mass of a single port
plus the acoustic mass of the air space between the flanges 416 and
418, plus appropriate end corrections. For a given diameter D1 of
the port tubes 404 and 408, the acoustic mass of the port structure
can be conveniently adjusted by varying the separation distance S
or the outer diameter D2 of the flanges 416 and 418. Increasing the
flange outer diameter D2, or decreasing the separation distance S,
leads to an increased total acoustic mass and a lower tuning
frequency. Thus, the port structure of the present invention
achieves greater acoustic mass in a more compact arrangement than
using multiple conventional ports such as shown in FIG. 2.
Referring to FIG. 3, which is a reproduction of FIG. 7 of U.S. Pat.
No. 5,517,573, a complete woofer system incorporating a preferred
embodiment of the '573 patent is shown. In FIG. 3, an enclosure 33
is provided with a partition 34 separating the interior of the
enclosure into a sealed chamber 36 and a vented chamber 37. As
shown in FIG. 3, two drivers 38 and 39 are mounted in the partition
34. A port opening 41 is provided to chamber 37 with a port or vent
tube 42 extending from the opening 41 back into the interior of
chamber 37. Disposed to either end of the port or vent tube are
disks or baffle plates 43 and 44 having associated flow directors
45 and 46. Connecting the flow directors and extending through the
vent tube is a connector 47. Accordingly, the method disclosed in
the '573 patent utilizes disc 43 and flow director 45 to create an
increasing cross-sectional area at the inside end of single port
tube 42.
In contrast and referring to FIG. 4, the present invention uses a
pair of flanges 416 and 418 at the ends of two opposed port tubes
404 and 408 to create an increasing cross-sectional area at the
inside end of the port structure. The larger radiating area of the
combined front and rear port openings 406 and 410, and the larger
combined cross-sectional area of the two port tubes has advantages
in further reducing turbulence and loss at the outer ends and gives
this port structure a unique bipolar radiation pattern. The
cross-sectional area of the space between the flanges 416 and 418
at opening 420 is equal to .pi.*D2*S and is greater than the
cross-sectional area between the flanges at the inside opening 422,
which is equal to .pi.E*D1*S. Therefore, the effect of the port
structure of the present invention as shown in FIG. 4 is to provide
a duct with a cross-sectional area which increases from some
minimum value to a larger value at opening 420 of the port
structure and functions similarly to a flared port, as shown in
FIG. 1 or U.S. Pat. No. 5,809,154, to reduce turbulence and loss.
Due to their shorter wavelengths, midrange and higher frequencies
generated inside enclosure 400 tend to pass through the air space
between flanges 416 and 418 without entering port tubes 404 and
408. Therefore, the transmission of these higher frequencies from
inside enclosure 400 to outside is reduced. Organ pipe resonances
typically occur at a lowest frequency whose wavelength is
approximately twice the length of a tube open at both ends. In the
present invention the two port tubes 404 and 408 are separated at
their inside ends by a predetermined separation distance S. This
separation distance substantially eliminates any resonance
associated with the combined length of the two port tubes and moves
the lowest organ pipe resonance upward more than one octave to a
frequency whose wavelength is approximately double the length L of
one port tube 404 or 408. This higher frequency resonance is less
likely to be audible and, due to the same mechanism which
suppresses transmission of unwanted midrange, is less strongly
excited by acoustic energy inside enclosure 400. The port structure
of FIG. 4 also offers a novel cosmetic appearance in the illusion
of an unobstructed open duct passing entirely through the
loudspeaker enclosure.
In a first preferred embodiment of the present invention, the
system Thiele-Small parameters are approximately as follows:
BL=12.6 weber/meter
Cms=0.000487 meter/newton
Sd=0.0368 sq. meters
Re=3.6 ohms
Mmd=0.1065 kg
Qms=5.5
fs=37.6 Hz
fc=45.6 Hz (the resonant frequency of the transducers when mounted
in the enclosure)
V=60.5 liter (the enclosure volume)
fp=45.6 Hz (the tuning frequency of the port)
where BL is the driver motor force factor; Cms is the compliance of
driver suspension; Sd is the driver cone area; Re is the driver
voice coil DC resistance; Mmd is the moving mass of the driver; Qms
is the mechanical Q of the driver; fs is the free-air resonance of
driver; fc is the resonant frequency of the transducers when
mounted in the enclosure; V is the enclosure volume; and fp is the
tuning frequency of the port.
Referring to FIG. 4, an example of the port structure dimensions
for this first preferred embodiment may be:
D1=4 inches
D2=6.5 inches
S=2 inches
L=6 inches
Experiments have shown that a system constructed in accordance with
this first preferred embodiment of the present invention has
significantly less vent noise and greater low frequency output than
a similar system utilizing the conventional methods disclosed in
U.S. Pat. Nos. 5,517,573 and 5,809,154.
Many variations are possible utilizing the basic principles of the
present invention. For example, a flare 106 such as shown in FIG. 1
may be added to one or both of the outer ends of port tubes 404 and
408 of FIG. 4 to further decrease turbulence and loss. In a further
example, and referring to FIG. 5, discs 502 and 504 may be added at
one or both of the outer openings 406 and 410 of port tubes 404 and
408, respectively, at a predetermined distance S2, according to the
teachings of U.S. Pat. No. 5,809,154 to provide an increasing
cross-sectional area at the outer ends of the port structure for
reduced turbulence and loss. Additional porting efficiency may be
achieved by adding flow guides 506 and 508, according to the
teachings of U.S. Pat. No. 5,517,573. Referring to FIG. 6, further
improvements in porting efficiency may be achieved by the addition
of a flow guide 602 centrally located between flanges 416 and
418.
Referring again to FIG. 4, it is generally desirable that the
separation distance S is selected such that the cross-sectional
area of the duct where the port tubes join the inside diameter of
the flanges at opening 422 and defined as .pi.*D1*S, is
approximately equal to the combined cross-sectional area of the two
port tubes 404 and 408, defined as 2*.pi.*(0.5*D1).sup.2. However,
it may be desirable to choose a smaller or larger value for the
separation distance S so as to adjust the acoustic mass of the port
structure to achieve the desired tuning frequency. Experiments have
shown that the porting method of the present invention is effective
for values of the separation distance S significantly less than
one-half diameter D1 to values of separation distance S greater
than twice diameter D1. For values of the separation distance S
outside this range the effectiveness of the porting method of the
present invention may be reduced. However, the unique benefits of a
bipolar radiation pattern, large total cross-sectional area and
novel appearance are maintained regardless of the separation
distance S or the diameter D2 of flanges 416 and 418 of FIG. 4, and
should be understood to fall within the scope of the present
invention.
It is also generally desirable for the two port tubes 404 and 408
to be substantially identical. However, practical considerations
may suggest the use of port tubes with different cross-sections,
different lengths and different acoustic masses. It will be
understood that this implementation is also within the scope of the
present invention and achieves the previously discussed benefits.
Similarly, it is not necessary for the port tubes 404 and 408 to be
of round or circular cross-section, or that the flanges 416 and 418
be circular or round in shape. Various cross-sectional shapes for
the port tubes 404 and 408 may be employed or various shapes chosen
for the flanges 416 and 418, while adhering to the basic principles
of the present invention, such as rectangular, square, triangular,
or other shapes. It is also not necessary for the loudspeaker
enclosure to be rectangular or of any particular shape so long as
the port structure is constructed in accordance with the principles
of the present invention disclosed herein. By way of example and
not of limitation, the loudspeaker enclosure could be of
cylindrical or rounded form with a port opening on one curved
surface and another port opening on an opposite curved surface.
Those skilled in the art will also understand that other variations
may be employed while remaining within the scope of the present
invention.
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